1. Field of the Invention
This invention relates to a recording apparatus for using a stepping motor as a driving source for a carriage mounted with recording means or as a driving source for carrying a recording medium.
2. Related Background Art
In a conventional serial type printer, a hybrid type or PM type (permanent magnet type) stepping motor is used as a motor for driving a carriage to move a recording head mounted on a carriage during recording. In such a case, the carriage is secured on a carrying belt, and this belt is rotated by a pulley installed to the rotation shaft of the stepping motor. The carriage is so constructed as to scan in the recording direction for recording by the movement of the belt attendant upon the rotation of the motor.
Such a stepping motor is synchronized with an external pulse, and switches the exciting current to the motor exciting phase to drive it for rotation. If the exciting current is switched faster than a specified value, the rotation of the stepping motor rotor is slightly delayed, and an error step called "step-out" occurs. Such a step-out throws the movement of the recording head into disorder, causing position slippage or an image disturbance of the recorded image.
Especially when the carriage is accelerated, it should be noted that step-out tends to occur because the load due to inertia is great in addition to unstable rotation of the stepping motor.
A motor control device, which closed loop controls a conventional stepping motor to solve such a problem, has been disclosed by U.S. Pat. No. 4,963,808, and further such a stepping motor used for a printer was previously proposed by U.S. Pat. No. 4,928,050.
That is, in a recording apparatus for using a stepping motor as a driving source and moving a recording head to scan for recording, this proposal is a recording apparatus comprising detecting means for detecting the position of the angle of rotation of the stepping motor rotor, and control means for closed loop controlling of the drive of the stepping motor in accordance with a detection result of the detecting means.
It has been proposed to solve the problem by open loop controlling, and not by closed loop controlling the drive of the stepping motor in this way. However, the open loop control of the stepping motor has further had the following problems.
Since a stepping signal is applied to each exciting phase to rotate the rotor in the stepping motor, when the stepping motor is rotated at a low-speed, the rotation of the rotor generates vibration, causing noise. For example, FIG. 18A is a view showing the relationship between rotary speed and time when the stepping motor has been rotated with the speed curve taken in a straight line so that acceleration is constant. In this case, vibration occurs in the rotation of the stepping motor causing noise as shown by numeral 80 when the driving frequency is about 100 Hz, the resonance frequency of the stepping motor.
To prevent noise due to vibration during such an acceleration, it has also been considered to rapidly increase the rotation frequency of the stepping motor to a specified number of revolutions. However, when the stepping motor is rotated at a specified speed higher than the resonance frequency of the stepping motor from the beginning, step-out occurs, and therefore this cannot be used. Accordingly, a desired rotary speed should be reached by accelerating after starting the rotation at a sufficiently low speed at the beginning.
For this reason, the stepping motor is conventionally driven for rotation so that the speed variation from the start to a constant speed follows an exponential curve in order to rotate the stepping motor without causing both step-out and noise.
In the above conventional example, however, the stepping motor rotor rotates late for a driving pulse applied to the motor when the carriage is accelerated for driving as mentioned above. For this reason, during drive at a constant speed, in which the recording operation is started after acceleration, overspeed (overshoot) or speed variation, etc. occurs. This does not allow the carrying speed of the recording head to reach a specified constant speed even at a position where the recording operation is started, and the recording precision is lowered.
FIG. 18B is a view showing the speed variation of the carriage (recording head) when the stepping motor was driven for rotation in accordance with a conventional exponential curve, and numerals 81 and 82 show portions in which the overshoot and speed variation occur respectively.
If a speed variation occurs because of such an overshoot, etc., when vertical ruled lines, for example, are recorded over the entire recording area in both directions recording (recording is performed when the carriage moves forward and returns), the recording position of vertical ruled lines at both ends of the recording area slips by the motion of the recording head, and as a result, each of them may not be recorded as one line.
This is because, since the direction of recording is in both directions, when the carriage moves in the right direction, for example, a vertical ruled line at the right end is recorded after the speed of the recording head has become fully a constant speed, while a vertical ruled line at the left end is recorded immediately after it is accelerated. On the other hand, when the recording head moves in the left direction, a vertical ruled line at the left end is recorded at a constant speed, while a vertical ruled line at the right end is recorded immediately after acceleration.
To prevent such a chaotic recording operation the time from when the acceleration is over until the recording operation starts is made sufficiently long, so that the start of the recording operation occurs after these overshoots, etc. have ended. However, the recording time will be long because of an extra operation until a constant speed is reached since the acceleration is finished, and the width of the recording apparatus will be greater than necessary to secure such a distance.